justloveplanes
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Electric Hum On 787

Mon Mar 12, 2012 8:35 pm

Several trip reports on the 787 mention that cabin is quiet, save for a noticable electric hum. Any thoughts on the origin of this hum and how it might be reduced?
 
yeelep
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RE: Electric Hum On 787

Mon Mar 12, 2012 9:39 pm

Just guessing, fuel pumps and airconditioning pack motors.
 
roseflyer
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RE: Electric Hum On 787

Mon Mar 12, 2012 10:05 pm

It really depends on where in the cabin you are would be my guess. I haven't flown on a 787, so I don't know that airplane well, but the loudest hum on a narrowbody is typically from the hydraulic motors if you are near the wheel well or the ventilation system fans if you are near the Electronics bay.
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yeelep
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RE: Electric Hum On 787

Tue Mar 13, 2012 12:00 am

Quoting Roseflyer (Reply 2):
ventilation system fans

Do you mean equipment cooling fans(Boeing's term)?

I thought the 787 had on demand electric hyd. pumps. Googled it and found they have constant and on demand electric hyd. pumps.

Another set of fans that can be heard are the cabin temperature sensor fans, at least on the 737.
 
tdscanuck
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RE: Electric Hum On 787

Tue Mar 13, 2012 12:05 am

Quoting justloveplanes (Thread starter):
Any thoughts on the origin of this hum and how it might be reduced?

The 787 has a lot more electrical power than comparable aircraft and there's an extra electronic equipment bay (aft of the wheel wells). As a result, there's a lot more electrical power running around in fairly close proximity to the passenger cabin.

Quoting yeelep (Reply 1):

Just guessing, fuel pumps and airconditioning pack motors.

Fuel pumps are pretty hard to hear on a widebody and the ACM goes so fast that what you hear is more air noise than the rotors themselves. I suspect most of the "electrical hum" is, in fact, electronics. The 787 has solid-state electronics all over the place thanks to the distributed more-electric architecture.

Tom.
 
CM
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RE: Electric Hum On 787

Tue Mar 13, 2012 12:30 am

Quoting justloveplanes (Thread starter):
a noticable electric hum

The 787 has two fuselage-mounted electric motor hydraulic pumps which power the center hydraulic system. They are on the bottom of the fuselage in the WTB fairing, and can be heard in the cabin when they operate. They make a buzzing noise, which may sound to some like an "electric hum". It wouldn't be constant however, generally only departure and approach.
 
spudsmac
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RE: Electric Hum On 787

Tue Mar 13, 2012 5:01 am

Quoting CM (Reply 5):

The 787 has two fuselage-mounted electric motor hydraulic pumps which power the center hydraulic system. They are on the bottom of the fuselage in the WTB fairing, and can be heard in the cabin when they operate. They make a buzzing noise, which may sound to some like an "electric hum". It wouldn't be constant however, generally only departure and approach.

What's the point of those? I thought the 787 was supposed to be all electric with regards to control surfaces and landing gear.
 
yeelep
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RE: Electric Hum On 787

Tue Mar 13, 2012 5:43 am

The primary flight controls, nose gear steering, landing gear, thrust reversers, and the trailing and leading edges are hydraulically driven. Some of the spoilers and the brakes are electromechanical.

The real change was eliminating most of the bleed air system.
 
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HAWK21M
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RE: Electric Hum On 787

Tue Mar 13, 2012 9:56 am

Quoting spudsmac (Reply 6):
I thought the 787 was supposed to be all electric with regards to control surfaces and landing gear.

Wasn't it supposed to be 5000psi Hydraulics operated instead.....
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Rara
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RE: Electric Hum On 787

Tue Mar 13, 2012 12:00 pm

Quoting yeelep (Reply 7):

The real change was eliminating most of the bleed air system.

What aspects remain?
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Lemmy
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RE: Electric Hum On 787

Tue Mar 13, 2012 1:41 pm

Quoting Rara (Reply 9):
What aspects remain?

Nacelle anti-ice, I think.
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mmedford
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RE: Electric Hum On 787

Tue Mar 13, 2012 6:32 pm

How about transformer hum?
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Northwest727
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RE: Electric Hum On 787

Tue Mar 13, 2012 9:18 pm

As the 787 gains more time in service, more videos are bound to appear. But from the few that are on YouTube, I can hear the whine myself. Here is a good example, with the engine noise in the background changing to show that the whine is not from the engines:

http://www.youtube.com/watch?v=-0uwUlKT8AI&feature=related

Does the 787 use typical fluorescent "flood lighting" (not the cool color-changing mood lighting we've all seen pictures of in Boeing marketing material) or is it completely 100% LED based? I know that the florescent lamps tend to emit a buzz on most conventional aircraft, not sure if it is from a ballast or something else.

Edit: on second thought, that video clearly shows the whine get more "higher pitched" as the engine spools up. Don't think I've ever heard florescent bulbs do that, so must be something else then I guess.

[Edited 2012-03-13 14:23:22]
 
flashmeister
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RE: Electric Hum On 787

Tue Mar 13, 2012 10:45 pm

Quoting Northwest727 (Reply 12):
Does the 787 use typical fluorescent "flood lighting" (not the cool color-changing mood lighting we've all seen pictures of in Boeing marketing material) or is it completely 100% LED based?

I believe that 787 is entirely LED-lit.
 
tdscanuck
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RE: Electric Hum On 787

Wed Mar 14, 2012 4:10 am

Quoting yeelep (Reply 3):
Quoting Roseflyer (Reply 2):
ventilation system fans

Do you mean equipment cooling fans(Boeing's term)?

They're not exactly the same thing. Equipment cooling fans pull air through the flight deck panels, displays, and equipment bays. Ventilation system fans pull air from the galley/lavs and pull recirc air back into the ECS system.

Quoting yeelep (Reply 3):
I thought the 787 had on demand electric hyd. pumps. Googled it and found they have constant and on demand electric hyd. pumps.

The L and R systems have one engine-driven pump and one electric-demand pump each...those electric pumps only come on when needed. The C system has two electric-motor pumps. One always runs, the other acts as a demand pump (they swap who's doing which roll every day).

Quoting spudsmac (Reply 6):
What's the point of those? I thought the 787 was supposed to be all electric with regards to control surfaces and landing gear.

yeelep is right, the 787 has a mostly conventional hydraulic system (albeit at higher pressure). It's very similar to a 777 in that regard. Landing gear actuation is still hydraulic. All control surfaces except the stabilizer and four spoilers are hydraulic (the stab and four spoilers are electric).

Quoting HAWK21M (Reply 8):
Wasn't it supposed to be 5000psi Hydraulics operated instead.....

It is.

Quoting Lemmy (Reply 10):
Quoting Rara (Reply 9):
What aspects remain?

Nacelle anti-ice, I think.

Yes. The 787 has no pneumatic system in the sense that there is no pneumatic supply from the engine to the airplane (there is no duct in the strut). The engine itself uses bleed air for several purposes (turbine cooling, turbine clearance control, valve actuation, nacelle anti-ice, etc.).

Quoting Northwest727 (Reply 12):
Does the 787 use typical fluorescent "flood lighting" (not the cool color-changing mood lighting we've all seen pictures of in Boeing marketing material) or is it completely 100% LED based?

All LED, except for some HID on the exterior lamps.

Quoting Northwest727 (Reply 12):
Here is a good example, with the engine noise in the background changing to show that the whine is not from the engines:

http://www.youtube.com/watch?v=-0uwU...lated

That's the core whine (much higher pitched and "leading" the fan) but it's still engine noise. Keep in mind that the 787 (and A380) use variable-frequency generation, so any noise directly associated with AC frequency in the electrical bits will also scale with engine speed.

Tom.
 
yeelep
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RE: Electric Hum On 787

Wed Mar 14, 2012 5:19 pm

Quoting tdscanuck (Reply 14):
They're not exactly the same thing. Equipment cooling fans pull air through the flight deck panels, displays, and equipment bays. Ventilation system fans pull air from the galley/lavs and pull recirc air back into the ECS system.

Tom,
What you describe sounds like recirculation fans minus the pulling air from the galleys/lavs. I suppose I'm a little confused by the term because my background is the 737 which does not have ventilation fans.
On the 737 equipment cooling fans also push air through the flight deck panels, displays, and equipment bays depending on whether they are supply or exhaust fans. Does the 787 only use exhaust type fans?

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